Early-exit validation: stop once enough valid proxies found

- validate_proxies: target= param — once N valid found, remaining
  tasks skip the HTTP call and drain immediately (no wasted time)
- _build_pool: target = max(min_pool_size, chain_length * 3)
- Settings: min_pool_size (default 20), max_candidates default 400->200
- UI: Stop after N valid / Max candidates fields in Settings tab
- README: updated settings table
- 1 new test for early-exit with unreachable target

Made-with: Cursor

proxy_chain_manager/service.py

@@ -579,12 +579,14 @@ class ChainService:
         def on_prog(done: int, total: int) -> None:
             self._notify({"type": "validate_progress", "done": done, "total": total})
 
+        target = max(s.min_pool_size, s.chain_length * 3)
         good = await validate_proxies(
             unique,
             s.ip_check_url,
             s.validation_concurrency,
             s.validation_timeout_seconds,
             on_progress=on_prog,
+            target=target,
         )
         random.shuffle(good)